Chromizing with heat treatment



Oct. 23, 1962 s. A. SAMUEL ETAL 3,059,911

CHROMIZING WITH HEAT TREATMENT Original Filed Nov. 6, 1958 ,2 Sheets-Sheet 1 Oct. 23, 1962 G. A. SAMUEL ETAL 3,059,911

CHROMIZING WITH HEAT TREATMENT Original Filed Nov. 6, 1958 2 Sheets-Sheet 2 INVENTORS Gad/ye A. Mme! ATTORNEYS trite tates if tet hfice 3,059,911 Patented Got. 23, 1952 pauy, line, Wilmington, Del, a corporation of Delaware 4 Claims. (Cl. 2664) The present invention relates to chromizing accompanied by heat treatment.

This application is a division of our copending application Serial No. 772,294, filed November 6, 1958, for Chromizing With Heat Treatment.

A purpose of the invention is to obtain bright chromized surfaces on non-air-hardening steels.

A further purpose is to permit the production of chromized steel parts which are harder particularly in the core.

A further purpose is to permit quenching of chromized steel parts from above the critical range by utilization of the heat of chromizing without serious damage to the chromized surface.

A further purpose is to obtain an improved surface condition of chromized steel parts which require hardeniug by quenching from the above critical range.

A further purpose is to quench chromized steel parts directly from the chromizing retort.

A further purpose is to rotate the chromizing retort and thus facilitate chromizing and more uniformly regulate the temperature in the parts and then quench the parts directly from the retort.

A further purpose is to provide an extension on the chromizing retort which is outside of the furnace and at relatively low temperature but is filled with chromizing gases, to bring the end of the extension into position beneath the surface of a quenching liquid, to open a door in the extension and to discharge the heated chromized steel parts through the extension and the open door into the quenching liquid while retaining the chromizing gas as a protecting gas in the retort.

A further purpose is to permit release of the door from a position above the quenching medium.

A further purpose is to hold the steel parts in the chromizing zone during chromizing by a baflle.

A further purpose is to separate the extension from the chromizing chamber by a plug which can be removed from outside the retort, and which retards but does not prevent gas from flowing into the extension, reduces heat losses and keeps the parts being chromized in the chromizing zone.

A further purpose is to purge the retort by an inert gas after quenching.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate a few only of the numerous embodiments in which the invention may appear, selecting the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

FIGURE 1 is a diagrammatic side elevation of the chromizing furnace and quenching tank according to the invention in chromizing position.

FIGURE 2 is a view similar to FIGURE 1 showing the chromizing furnace in quenching position.

FIGURE 3 is a fragmentary side elevation of the retort of the invention, omitting the furnace.

FIGURE 4 is an enlarged axial section of the central portion of the retort as shown in FIGURE 3.

Describing in illustration but not in limitation and referring to the drawings:

In the prior art chromized steel parts have usually been annealed as a result of the chromizing operation.

Efforts to reharden such parts using standard hardening techniques have met with limited success, because the hardening and quenching procedures have resulted in serious oxidation of the chromized layer. Heat treating has been attempted in controlled atmosphere furnaces, but this is very expensive, and widespread difiiculty has occurred even with atmosphere-controlled heating due to the tendency to produce a discoloration on the parts which is not readily removed, and which is indicative of damage to the chromizing layer.

In accordance with the present invention, the heat of chromizing is used for hardening, and the chromized steel parts are discharged to the quenching medium through the chromizing atmosphere which still protects the parts. As a consequence, it is possible to obtain according to the invention hardening and other similar heat treatments, without serious damage to the chromized layer. Thus it is possible in many steels to restore the strength imparted by cold work and otherwise lost in chromizing.

In the preferred embodiment of the invention, the chromizing retort is provided with an extension which is outside the chromizing furnace and maintained at a very low temperature, although it is filled with the chromizing or other protecting gas. This extension is used to convey the chromized steel parts to the quenching liquid, suitably by opening a door immersed beneath the level of the quenching liquid. The door is desirably controlled from above the surface of the quenching liquid.

In the preferred embodiment of the invention, a tilting furnace is used which is tilted up for chromizing and is tilted down to quench.

Considering now the drawings in detail, these illustrate a furnace support 2% having trunnions 21 which pivotally support a furnace 22, suitably a gas-fired furnace, having a furnace housing 23, burners 24 and a gas inlet connection 25 through a regulator valve 26 and branch pipes 27 which supply gas to the burners.

Within the furnace but sticking out from it there is a retort 28 which consists of a chromizing chamber 30 suitably of heat-resisting steel, best seen in FIGURES 3 and 4 and located inside the furnace, a front extension 31 open to the interior of the retort and a rear extension 32 located at the rear of the retort.

The retort is desirably of circular cross section throughout, and the rear extension 32 is suitably journaled on a bearing 33 at the rear of the furnace, conveniently made from a carbon (graphite) block which will resist the combustion gases, while the front extension is conveniently journaled on roller bearings 34 distributed around the lower portion of the circumference. By tilting the furnace so that the front extension 31 is at the top, and removing the gear 35 described infra, the retort can be picked up by a crane and lifted out of the furnace and a cold fully charged retort inserted in the furnace.

The rear extension is suitably tubular and has removably secured thereon a gear 35 which meshes with a driving pinion 36 on output shaft 37 of a driving electric motor and speed reducer 38 mounted on a bracket 39 extending from the furnace 22. Accordingly the entire retort is desirably rotated, suitably at a speed of 0.5 to 4 rpm, during the chromizing operation. Chromizing can be conducted without rotation if desired.

At the forward end of the tubular front extension 31 there is a gasket 40 suitably of rubber which seals the joint between the extension 31 and a cover 41 in a gas 3 tight manner. Synthetic rubber, polytetrafiuorethylene (Teflon) or silicon rubber may be used for the gasket.

At one side the cover is held in place by bolt 42 extending through the cover and through lug 43 welded on the side of the front extension 31. The bolt is drawn tight by nut 44.

At the other side the cover or door is drawn tight by nut 45 on stud 46 passing through a slot in the cover, the stud is anchored at the other end by latching lever 47 pivoted at 48 on the side of the front extension and operating as a quick toggle release. The stud 46 is guided by slotted lug 50 which is secured to the side of the extension. The lever 47 is pivotally connected to the end of the stud 46 and 51.

The chromizing chamber 30 is a suitably open container which is provided with a mass of steel parts 52 to be chromized, and a source of chromium 4 which is preferably mixed up with the work, as lump ferrochrome or chromium, but may be contained within one or more steel mesh bags 53 which are free to tumble around with the work, as shown.

The space inside the retort also is provided with a chrornizing catalyst, which is preferably a halide, such as ammonium halide, sodium halide, potassium halide, chromous halide, iron halide, or a wide selection of other halides as well known in the art. Where the catalyst does not develop a suificient vapor pressure to sweep out the air, a separate air clearing material should be used. In the case of ammonium chloride, fluoride or bromide, for example, the air is adequately cleared by the hydrogen and nitrogen formed on heating.

Between the chromizing chamber 30 and the front extension 31 the diameter of the retort is reduced by a funnel 55, which extends into the extension 31 forming a seat rim 56. A plug 57 having gas flow Openings 53, too small to allow the work or the ferrochrome to escape, is placed against the seat rim, desirably positioned by a perforated frame 60 which is connected through rod 61, flexible coupling 62 and rod 63 with the interior of the cover 41 so that when the cover is removed the plug is withdrawn by the same action.

In the space around the plug, a ring of refractory heat insulation 64 extends to protect the extension from the heat of the furnace.

Thus the outer end of the extension is relatively cool and in any case below the temperature of boiling water in normal operation.

The cover 41 is provided with an opening which connects with valve 65 which during chromizing is ordinarily open and connected to flexible tube 66 which extends down below the level of the quenching liquid, assuming it is water or oil.

A valve cock 67 is provided at the end of the rear extension and a valve cock 68 is provided at the side of the front extension for purging purposes, as later explained. As shown in FIGURE 2, a quenching tank 70 is placed below and preferably in front of the furnace, filled with quenching liquid 71 which is preferably oil or water, although in the case of interrupted quenches, molten salt baths may be used. The tank at the side toward the furnace has a sloping lip 72 and when the furnace is tilted downward, the lower end of the extension 31 and the cover are beneath the level of the quenching liquid, and far enough below (at least inches) to prevent entry of air. The operator can then Open the cover by opening the toggle fastener 47, in which case the cover falls away from the bolts 42 and 46 by reason of the bolts leaving the slots in the cover, and the plug 57 drops out with the cover into the bottom of the quenching tank, while the work discharges through the funnel into the quenching tank and collects in elevator basket 73.

In operation, the charge is initially placed in the cold retort when cover 41 and plug 57 have been removed. The charge is suitably pickled or otherwise cleaned. With the charge is placed ferrochrome or other source of chromium (suitably degreased) and the catalyst. The cover 41 is then adjusted and tightened, purge cocks 67 and 68 are closed, and the valve in connection 65 is opened, while the rubber tube 66 extends down below the surface of the quenching liquid. The retort is initially filled with air. The furnace is then started, heating up the chromizing chamber of the retort. This causes some of the chromizing catalyst to break down, and in the case of ammonium bromide, for example, nitrogen, hydrogen and hydrogen bromide are produced, and air is expelled through connection 65 and discharges beneath the surface of the quenching medium. Some of the catalyst volatilizes and condenses particularly in the forward extension 31 close to the inner end, and this is a desirable feature because it provides a delayed action since as the retort becomes more thoroughly heated some of this catalyst breaks down and assists in the reaction.

As soon as heating starts, the retort is preferably rotated by the motor 38, and this causes the steel parts of the work and the source of chromium to constantly tumble, preventing lumps of chromium or ferrochrome from adhering to the work and preventing work pieces from adhering to one another. The furnace is further heated until the portion of the retort where the work and the source of chromium and some of the chromizing catalyst are located reaches a temperature of 1650 to 2400 degrees F., preferably 1750 to 2100 degrees F. and most desirably 1800 to 1900 degrees F. As the tumbling action continues, the work and the source of chromium are confined in the chromizing chamber by the plug 57, but gases evolve more or less continuously through the tube 66. There may be a time reached at which gases cease to evolve and the tube 66 shows a tendency to suck in liquid. This can readily be seen by placing a short section of glass tube between two portions of the tube 66. if this happens the valve at 65 is closed and a vacuum develops in the retort.

The time at temperature will depend on the depth of chromizing desired and the temperature and other factors, but in many cases we obtain good results in 1 to 4 hours, although heats as long as 12 or 24 hours may be used especially where the retort is not rotating but remains stationary.

After chromizing is complete, we bring the retort into quenching position, and this is preferably accomplished by tilting the retort and furnace as shown in FIGURE 2 until the door or cover and the lower end of the front extension 31 are beneath the level of the oil or water or other quenching liquid by about ten inches. A11 important aspect of the invention is that the lower end of the extension is so cool that it does not violently act on the quenching liquid which would cause a considerable amount of air to mix with the quenching liquid by virtue of boiling, vaporizing or breaking down of the oil. Therefore there is no pronounced tendency of the quenching liquid itself to oxidize the work. Furthermore as will be seen in FIGURE 2, the work does not pass through the air in coming from the retort to the quneching liquid.

In order to quench it is merely necessary to pull the quick release fastener 47, thus releasing the cover 41 and the plug 57 which drop out, allowing the chromized steel parts to fall through the funnel 55 and the extension 31, while protected by the chromizing or other suitable gas, into the quenching liquid.

Of course it will be evident that suitable techniques may be employed as desired to agitate the work or the oil or other quenching medium, or both, and a stirrer 74 normally is used in the quenching medium.

As soon as the work has left the retort, the retort can be returned to horizontal position. As a safety measure, since the gas in the retort usually contains considerable amounts of hydrogen, it is best to connect hoses to purge cocks 67 and 68 from a source of inert gas such as nitrogen, and blow out the chromizing gas from the retort before the retort is raised to the position of FIGURE 1.

Example I An initial run with a new retort was made with scrap saw chain steel parts using the procedure of this example to condition the retort.

After this conditioning run had been completed and the charge removed, the following charge was placed in the retort:

70 pounds of flat conveyor chain links consisting of AISI 1040 hot rolled steel. The links were pickled and ball burnished.

150 pounds of inch lumps of ferrochrome having the following analysis: chromium 63 to 66 percent, silicon 2 percent maximum, and carbon 0.01 percent maximum, all by weight.

75 grams of ammonium bromide.

All percentages are by weight.

The retort was closed and the discharge tube 66 placed beneath the level of the quenching oil, which was the quenching liquid. The furnace was started and the retort rotated at 3.4 -r.p.m. The chromizing chamber of the retort was held at approximately 1800 degrees F. for 2 hours,

during which time activator gases discharged through the tube 66. Then the furnace and retort were tilted forward to the quenching position and the retort with its cover and extension immersed in the quenching liquid was opened, discharging work into 637 gallons of Sun Oil No. 11 quenching oil at least 5 feet deep in the quench tank at room temperature. The quenching oil has the following properties:

Specific gravity 31.0 to 33.0. Viscosity at 100 degrees F 100 to 120 centipoises. Flash point 370 degrees F.

Fire point 420 degrees F. Carbon residue (Conradson) 0.01% maximum.

Mineral oil base.

The oil was agitated. The work discharged very rapidly into the quenching bath without exposure to air.

The chromized layer on test proved to be normal, rich in chromium carbide, extending to a depth of 0.0015

inch. This case resisted attack by boiling 20 percent nitric acid, resisted abrasion by a hardened steel scribe and scratched glass.

The steel parts had a core hardness averaging Rockwell C 42 to 45. Ninety percent of the parts were bright and clean as quenched. The remainder showed a slight rainbow hue which disappeared after bufiing.

Example 11 Example III In this example the following charge is used, all other conditions being the same as in Example I.

60 pounds of flat conveyor links made from AISI 1040 hot rolled steel, previously pickled.

showed various degrees of tinting.

175 pounds of inch ferrochrome lumps of the coniposition mentioned above.

70 grams of ammonium bromide activator.

The quenching oil (same as above) in this case was not agitated. The results were identical with those of Example I except that the parts had a rainbow hue, which was removed by butfing.

Example IV Here the charge was as follows:

30 pounds of AISI 1012 low carbon resulphurized free machining steel nuts for strip heaters, previously pick'led.

190 pounds of A lump ferrochrome.

70 grams of ammonium bifiuoride.

All the procedure was that of Example III except that the retort was held at 1900 degrees F. for 2 hours. Unlike the previous examples, the rotation of the retort was stopped just before the door or cover was opened so as to slow down the discharge of the work into the quenching medium by a matter of a few seconds. Midway in the discharge cycle the rotation of the retort was restarted momentarily. Approximately percent of the parts showed the characteristic bright gray chromized finish of low carbon steel, but the remainder The case depth was 0.002 inch, and was fairly ductile. The case continuity was excellent as revealed by immersion in boiling 20 percent nitric acid.

The parts before chromizing had a hardness of Rockwell B indicating the effect of cold work, and after quenching their hardness averaged Rockwell B 67.

Example V Into a smaller retort of the general character previously described, the following charge was placed:

/2 pound chain saw steel parts AISI 1070.

5 pounds of A inch lumps of ferrochrome as above.

10 grams of ammonium bromide.

The work was chromized at temperature for 1 hour at 1950 degrees F. The work was directly quenched into oil. The case was clean and not oxidized and thin. The core had a hardness of Rockwell C 63.

If desired and if the heat treating cycle permits, the work can be cooled somewhat in the retort before quenchmg.

It will be evident that the retort gas, given off as bubbles during quenching, protects against explosion, since even if the bubbles burn they are not confined.

Oil quench is preferred to water quench, as in the case of water quench, steam is generated which is slightly oxidizing.

It will be evident that by using lump ferrochrome or chromium (of a size between A and /2 inch diameter, preferably about inch) it is possible to quenclr the ferrochrome or chromium with the work and then degrease the ferrochrorne or chromium and rinse it without damage. Where powdered ferrochrome or chromium is used, this is not practical, as the powder is very reactive with the quenching liquid. It will also be evident that if desired the source of chromium can be prevented from entering the quenching liquid, as by interposing a screen having mesh small enough to catch the wire mesh bags of ferrochrome or chromium, but passing the work.

In view of our invention and disclosures, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of our invention without copying the structure shown, and we, therefore, claim all such insofar as they fall within the reasonable spirit and scope of our claims.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. A tubular retort for chromizing and the like comprising a tubular furnace, means for the furnace to be tiltably engaged with a support, a processing chamber having a processing zone therein within the furnace for receiving work to be processed, and a tubular extension of said chamber having an outer end, said extension being exposed all the way to said outer end thereof on the inside to the atmosphere within the processing zone and projecting out beyond the furnace and exposed on the outside to the air and cooled by the air, said tubular extension having a smaller outside diameter than the diameter of the processing chamber and free from external insulation to promote heat transfer from the extension, and said tubular extension having a door for said outer end, selectively for said inside of the tubular extension to be closed off from the air and opened at said outer end, and said tubular retort further comprising thermal insulation in the interior where the extension joins said chamber reducing the loss of heat from the processing zone and from the furnace into the extension and thus further aiding in keeping the extension cool, the extension when the furnace is tilted being adapted to submerge said outer end directly beneath a quenching medium and to transmit, upon said outer end being opened, work from the processing zone to the quenching medium without passing the work through the air.

2. Equipment for chromizing and the like comprising a processing retort having a processing zone therein for receiving work to be processed, heating means adjacent to said retort for the retort to be heated, a tubular extension of said retort having an outer end, means for said retort and said extension to be tiltably engaged with a support, said tubular extension of the retort being exposed all the way to said outer end thereof on the inside to the atmosphere within the processing zone and and said equipment further comprising thermal insulation in the interior where the extension joins said retort reducing the loss of heat from the processing zone and from said heating means into the extension and thus further aiding in keeping the extension cool, the extension when said retort and extension are tilted being adapted to submerge said outer end directly beneath a quenching medium and to transmit, upon said outer end being opened, work from the processing zone to the quenching medium without passing the work through the air.

3. An equipment of claim 2, wherein a perforated plug is connected to said door and is within said extension to prevent work in the processing zone from entering the extension during processing, said perforated plug permitting atmosphere from the processing zone to enter the extension and fill the extension with the atmosphere, and said plug being movable axially of said extension and out of said outer end of the extension when said extension is opened at said outer end.

4. An equipment of claim 3, in combination with a valve flushing gas outlet from the inside of said extension and a second extension from said retort having a flushing gas inlet to said processing zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,445,220 Lee Feb. 13, 1923 2,240,863 Scullen May 6, 1941 2,273,991 Randolph et al. Feb. 24, 1942 2,290,551 Grier July 21, 1942 FOREIGN PATENTS 343,306 Great Britain Feb. 19, 1931 560,222 Germany Sept. 29, 1932 573,134 Germany Mar. 9, 1933 494,340 Great Britain Oct. 24, 1938 256,598 Sweden Mar. 1, 1949 796,088 Great Britain June 4, 1958 821,469 Great Britain Oct. 7, 1959 

1. A TUBLAR RETORT FOR CHROMIZING AND THE LIKE COMPRISING A TUBULAR FURNACE, MEANS FOR THE FURNACE TO BE TILTABLY ENGAGED WITH A SUPPORT, A PROCESSING CHAMBER 